Graphene represents a form of carbon in which the carbon atoms reside within a single atomically thin sheet or a few layered sheets (e.g., about 20 or less) of six-membered lattice rings. One known method of producing high quality, large-scale graphene sheets (i.e., 1 cm2 or larger) is through chemical vapor deposition (CVD). During CVD, a growth substrate is exposed to one or more gaseous reactants, which react to deposit a carbon film on the surface of the growth substrate, resulting in the production of a graphene sheet. After growth, the graphene sheet must then be transferred to a functional substrate suitable for the intended application of the graphene sheet. To transfer the graphene sheet to the desired substrate requires separation of the graphene sheet from the growth substrate, which may result in tearing, cracking, or other substantial defects in the graphene sheet, especially in large-scale transfers in which the risk of damage is higher. In general, two methods may be used to facilitate the transfer of the graphene sheet from the growth substrate: the supported transfer method and the free-float transfer method.
The supported transfer method typically involves the use of a support polymer, such as poly(methyl methacrylate) (PMMA) or other similar polymers. In this method, the graphene is coated with PMMA and then the underlying growth substrate is etched away. The PMMA-graphene composite is then transferred to the functional substrate and mounted. Once mounted, the composite is washed with a solvent to remove the PMMA. Because this method provides a physical support to the graphene during transfer, large-scale transfer of graphene sheets is made possible. However, the use of the polymer leaves contaminants or residues on the surface of the graphene sheet. While it is possible to remove the PMMA such that the contaminants or residues are present in small amounts, even small amounts may nevertheless impact the quality of the sheet. This impact in quality, however small, may be significant in certain applications. For example, the contaminants or residues may impact the ability to reliably perforate the graphene sheet. In addition, the solvent required to remove the polymer may limit the type of functional substrate that may be used. For example, in removing PMMA, acetone is typically used. The use of this solvent, however, may prevent the use of track-etched polycarbonate as a functional substrate.
The free-float transfer method typically requires floating the graphene in a solution. During this method, the graphene-growth substrate composite is first floated in an etching solution containing an agent that etches away the growth substrate, producing a free-floating graphene sheet. The etching solution is then washed out and changed to a water-based solution to allow the graphene to be floated onto the desired substrate. As the free-float transfer method does not involve the use of secondary polymer materials to coat the graphene sheet, the free-float transfer method is desirable over the supported transfer method due to the decreased risk of introducing contaminants or leaving residue on the graphene sheet. However, large-scale transfer of the graphene sheet is difficult using this method as the risk of tearing or otherwise damaging the sheet is higher due to the unsupported nature of the transfer method.